Don't know how much help this will be but here goes....
Re: AE TCs, we bring them ALL the time and I still consider it to be the safest way to transport cash without the concerns of carrying green cash,and we've only had one problem in all our forty years. We ate at Maximo's at Porto Cupecoy last April and they would NOT accept them. But a few words to the wise - travelers checks are easily cashed at the casinos or major grocery stores with photo id. Gas stations as a rule do NOT accept anything but CASH. We bring hundred dollar denominations and use them primarily for dinners, groceries, and shopping expeditions, usually ending up with enough cash for the next day without actually worrying about breaking a check but that said, a lot of people are more comfortable with smaller denominations. I can't stand writing my names all those times....
And in conjunction with the TCs, we usually bring two credit cards, one for our rental car and hotel imprint and the other kept in our safe for emergencies, should they arise and then about $500. in cash in smaller denominations for tips and drinks and settling in cash. We don't use the ATMs because over the years there have been a lot of problems reported although I know many people today depend on CCs and ATMs.
I don't know that much about Guana Bay beyond the surf there can be rough so I'm not sure the snorkeling in that area would be any good. That said, take a ride to Dawn Beach, Orient, Rouge or Mullet and you'll have some of the better island snorkeling, or so I'm told. My daughter and SIL always bring their masks and fins with them but I know they can be rented, usually by the day or the hour. But again, if you're already shipping stuff.....you know your own gear.
Never used a Tracphone but I'm doubting it will work there. Most people will rent there island cell phone from Sharon, Queen of St. Maarten - great rates and incredibly good service and they work on both sides of the island.
If you're bringing sunscreen you're probably shipping luggage so yes, I'd bring a Deet based repellent with me. You can easily get both on the island but if you're already shipping stuff, might just as well bring it from home and save yourself having to look for it on the island. The worst times of day for critter bites is usually late afternoon and early evening and the little buggers can do a real number on your ankles. I also keep the OFF towelettes in my purse and beach bag jic I need them during the day. If it's breezy you won't have as much of a problem but if it's humid, look out!!
You can pick up a fairly decent road map at Rima's or the Shipwreck Shops if you're sure you're going to need one. We find SXM to be a small and relatively easy island to explore as long as the traffic isn't real heavy and when it is, we generally turn around and head in the opposite direction and eventually we get to where we had hoped to get to - and even if it takes us a little longer, the exploring is usually worth the effort. It's amazing the incredible places we've discovered 'by mistake'.
My one warning is leave nothing in your car, not even locked in the trunk or glove compartment, if it's of any importance to you. Sadly, the big island negative is that car break-ins have become a bit of an island 'cottage industry' and it can happen in minutes when a car is left in a convenient spot. We are usually told by our rental company to always keep the car locked, except when we are at the beach - saves the creeps who would the damage they might inflict on your car if they have to break the locks first to check it out.
And likewise, if your rental has a room safe, it really is a smart move to keep your cash and jewelry in it.
Have a wonderful trip....we go every year once or twice a year and have for a long time - it's a great island with nice people, beautiful beaches, incredible food and a place where, with a little bit of caution and good sense, you can have an awesome time.